Development of electrostatic images using a developer that does not include a carrier has become popular in low-cost printers and copiers. This system does not require a toner monitor. The rest of the mechanism for applying toner is generally less expensive than that used in two component systems. This approach provides substantial development latitude of textural subject matter but is generally not able to reproduce gray scale or continuous tones reliably.
A typical development device using single-component nonmagnetic developer includes a developer roller which is rotated to move in the same direction as an electrostatic image to be developed. A toner-adder roller both applies toner to the developer roller and charges that toner utilizing both a difference in applied potential between the toner-adder roller and the developer roller and substantial triboelectrical effect from rubbing contact between the two rollers. A doctor blade controls the thickness of the layer of developer on the developer roller and can also be used to increase or control the charge on the toner. The developer roller contacts or nearly contacts the electrostatic image bearing number while developing the image under the influence of an electric field having DC or both DC and AC components.
U.S. Pat. No. 4,873,940 Ishikawa et al, issued Oct. 17, 1989, shows a developing device using single-component magnetic toner in which transportation of the developer is assisted by a magnetic development roller but in which charging of the toner is accomplished similarly to the nonmagnetic type of system. In this system the bias between the toner-adding roller and the development roller can be switched in order to make the toner positive or negative with respect to the electrostatic image.
These systems are affected by changes in relative humidity which may account in part for their nonuse in gray scale toning applications.
U.S. Pat. No. 4,521,103 shows an attempt to compensate for problems in a single component magnetic development system by varying the bias on a developer roller with respect to the electrostatic image as a function of measured conditions, including temperature and relative humidity.
Control of the bias on the development roller with respect to the electrostatic image as a function of relative humidity can help prevent problems at one end of the exposure scale or the other depending on whether the humidity is high or low and may be an acceptable compensation in developing text material. However, such a system is still inadequate when toning "gray scale images" in which there is interest in toning potential differences across a full range of potential.